The RW4 was derived from the high-wing RagWing RW1 Ultra-Piet as an FAR 103 Ultralight Vehicles compliant aircraft that would have an empty weight within that category's 2540NaN0 empty weight limit. The RW4 was designed to be an ultralight replica of the classic 1931 vintage Church Midwing Sport Monoplane pylon racer.
The RW4 airframe is constructed entirely from wood and covered with aircraft fabric. The landing gear is of conventional configuration with bungee suspension. As on the original Church design the RW4 has a gap between the wing and fuselage to allow the pilot to see downwards. The aircraft's installed power range is 20to and the standard engine was originally the 300NaN0 Kawasaki 340. The 350NaN0 Half VW four-stroke engine, 280NaN0 Rotax 277 two-stroke and 280NaN0 2si 430 two-stroke have also been used.
Partial kits and construction kits were available in the late 1990s, but today the aircraft is offered only in the form of plans. Reported construction time is 400–600 hours, although the designer claims 350 hours.